Limited Slip Differential

[john.jay@hotmail.com]

Can anyone explain how a LSD works in corners?

[Qualified Pole]

Can anyone explain how a LSD works in corners?

A limited slip differential allows the drive wheels to put power down through and out of corners and improves control by ensuring that torque is applied to both rear (in a GT3) wheels. To understand how and why an LSD works, it is useful to first understand how an open diff works and why it is needed, then compare/contrast. Here is some reading:

Wikipedia link

Car Bibles link (go to differential section, 3/4 of way down)

[john.jay@hotmail.com]

Can anyone explain how a LSD works in corners?

A limited slip differential allows the drive wheels to put power down through and out of corners and improves control by ensuring that torque is applied to both rear (in a GT3) wheels. To understand how and why an LSD works, it is useful to first understand how an open diff works and why it is needed, then compare/contrast. Here is some reading:

Wikipedia link

Car Bibles link (go to differential section, 3/4 of way down)

:) Thanks Q.P. But now what do the LSD % quotes mean e.g. 40/60, etc. And how do these variation help me get through a corner?

[Qualified Pole]

:) Thanks Q.P. But now what do the LSD % quotes mean e.g. 40/60, etc. And how do these variation help me get through a corner?

By your re-statement, I think you are going to need more expertise/experience than I can offer. I am still trying to figure out why my compression and rebound setting changes frequently do not do what I think they should.

That said, I am reasonably sure (wait for a more knowledgeable responder or second opinion) that the percentages mean the amount of torque difference between the wheels on acceleration/deceleration. Obviously, you would want more difference allowed during deceleration.

There is a reasonably easy to understand differential tuning blurb here, but be sure to note that the number shown are not %ages, they are ramp angles. The higher the ramp angle, the greater the difference allowed.

Excellent technical article that helped me with understanding %ages.

Please share what you learn.

[john.jay@hotmail.com]

:) Thanks Q.P. But now what do the LSD % quotes mean e.g. 40/60, etc. And how do these variation help me get through a corner?

By your re-statement, I think you are going to need more expertise/experience than I can offer. I am still trying to figure out why my compression and rebound setting changes frequently do not do what I think they should.

That said, I am reasonably sure (wait for a more knowledgeable responder or second opinion) that the percentages mean the amount of torque difference between the wheels on acceleration/deceleration. Obviously, you would want more difference allowed during deceleration.

There is a reasonably easy to understand differential tuning blurb here, but be sure to note that the number shown are not %ages, they are ramp angles. The higher the ramp angle, the greater the difference allowed.

Excellent technical article that helped me with understanding %ages.

Please share what you learn.

Thanks QP. Article's great and given the late hour, I'm sure offers a cure for insomnia.... :)

[seymann]

I always understood that with the car on a lift, if you turned one wheel the other would turn the same way with a limited slip diff. I tried this with my GT3 and found that the other wheel didn't turn at all (or just barely). Is this normal with the GT3 diff or is something amiss.

[993_996_enthusiast]

I always understood that with the car on a lift, if you turned one wheel the other would turn the same way with a limited slip diff. I tried this with my GT3 and found that the other wheel didn't turn at all (or just barely). Is this normal with the GT3 diff or is something amiss.

That is not unusual for a used 996 or 997 GT3 differential. The original pre-load on the disc plates is quite low on the street GT3 model cars. The pre-load copper or brass material wears faster than what is found in GT3 Cup cars. The combined mass and rotational friction of the wheel/tire, brake disk, and drive shaft universal joint can be sufficient to keep the other wheel from rotating while you turn one by hand.

Edited February 16, 2009 by 993_996_enthusiast

[seymann]

I always understood that with the car on a lift, if you turned one wheel the other would turn the same way with a limited slip diff. I tried this with my GT3 and found that the other wheel didn't turn at all (or just barely). Is this normal with the GT3 diff or is something amiss.

That is not unusual for a used 996 or 997 GT3 differential. The original pre-load on the disc plates is quite low on the street GT3 model cars. The disc plate copper or brass material wears faster than the steel found in GT3 Cup cars. The combined mass and rotational friction of the wheel/tire, brake disk, and drive shaft universal joint can be sufficient to keep the other wheel from rotating while you turn one by hand.

Thanks.

[Brad Roberts]

Whoa.. :lol:

60/40

60% lock on acceleration and 40% lock on decel (or whatever the ramp config is meaning that we can change the ramps in the GT3 ZF style diffs to make it 40/60 or 80/20 or 20/80)

In the PRO racing world, we use the diff as a tuning tool for the driver. Some like more lock on decel, some don't! Some like 100% lock!

The clutch plate ZF diffs can be fully rebuilt and basically "repacked" to achieve different amounts of "lock". It is a simple combination of friction plates and steel plates. Depending on how you stack them as you re-assemble the diff will effect the amount of lock.

It goes like this:

friction-steel-friction-steel-friction-steel (say this is 50% lock)

If I stack them this way:

friction-friction-steel-steel-friction-steel (this combo would yield 25% lock)

Now throw in a different thickness on the steel plates (we call it packing the diff) now you can create more spring pressure or flip the "wave" washers over and lose compression on the frictions-steels :)

I'll go into "ramps" if you like :)

The picture below is what is inside your diff. Item number 6 will be the friction-steel packs.

B

I'm kinda bummed out.. I have pics of a ZF Cup Diff dis-assembled and laid out. I took the pics to document exactly how it came apart so we could put it back together with the exact amount of lock (we did this at the Thunderhill 25hr race a few years back) I called up Paul Guard and met him near his house to pick up new parts and rebuild our diff in the pit area :)

B

[BuzzedHornet]

I have just started my research on 997 GT3's and found this site. This is great info! Thank you for the detailed explanation.

[harmonyjim]

Keep in mind that the oem GT3 LSD really is poor quality - both in materials and in preload values.  Most GT3 differentials, when pulled apart, are barely making contact between the plates (no, or very low preload left), so in essence, they act like an open differential.  Guard can get you up to speed far better than I can, but jack your car's rear end up, and see if the opposite side wheel turns when you turn one side.  Many forum members who pay close attention to this have said there is no proper differential action when they do this test on their cars.  I had an entire set of Guard 60/40 guts installed in my oem LSD housing (parts = approx $1800), and the car seems far less skittish upon corner entry.  I don't race or track my RS, so I am wasting much of the advantages of a properly working LSD, but I have the comfort of knowing that it is properly functioning.